Acoustic guitars include a sound hole located in a sound board at a centered position in a guitar body and underneath strings of the instrument. Vibration tones from the strings are transmitted to the sound board by a bridge. This causes vibration sound waves in the air filling the resonant cavity of the guitar body. The sound waves are resounded and amplified in the resonant cavity and are emitted from the guitar body through the sound hole. Although acoustic guitars have been designed this way for a long time, the sound emitted from these guitars is monophonic and often unclear.